1. Field of the Invention
This invention relates to (1) a heat exchanger, and more particularly to a heat exchanger having fins and tubes that are used primarily, although not exclusively in the heating, ventilation, air conditioning and refrigeration (HVACR) industry; and (2) a method for improving the efficiency of such heat exchangers.
2. Background Art
The Department of Energy (DOE) announced on Apr. 2, 2004 that it will enforce a 13 seasonal energy efficiency rating “SEER” standard for residential central air conditioners. This regulation affects residential central air conditioners and heat pumps. After Jan. 23, 2006, equipment manufactured must make the 13 SEER standard. It increases by 30% the SEER standard that applies to models sold at this time. Accordingly, manufacturers face a significant challenge in meeting the deadline for the thirteen SEER standard within the time allotted. This change in government-mandated standards gives rise to a need for higher efficiency in heat exchangers.
Conventionally, fin and tube heat exchangers used in the HVACR industry are constructed from round copper tubes and aluminum fins. Heat transfer by conduction and convection occurs, for example, from a fluid such as air flowing through the aluminum fins and around the copper tubes to the refrigerant carried in the tubes. For heating applications, the heat exchanger may be constructed of stainless steel or other materials to manage high temperatures, thermal cycling, and a corrosive environment.
Traditionally, a fin collar base is provided upon the fin, through which an outside diameter of a tube passes.
It is also known that one factor which limits local convective heat transfer is the presence of thermal boundary layers located on the plate fin surfaces of heat exchangers. Accordingly, conventional fins are often provided with means for varying surface topography or enhancements that disturb the boundary layer, thereby improving efficiency of heat transfer between the fluid passing through the tubes and the fluid that passes over the plate fin surfaces.
In the case of fin and tube heat exchangers, it is known that using protrusions at critical locations on the fin surface adjacent to a tube will enhance airside heat transfer performance of the heat exchanger. The provision of louvers, for example, tends to reduce the thickness of the hydrodynamic boundary layer. They tend to generate secondary flows which increase the efficiency of heat transfer. But large numbers of louvers, if added to a surface to improve heat transfer, usually are accompanied by an increase in pressure drop through the heat transfer apparatus, which is—other things being equal—an undesirable consequence.
Louvers are provided by rotating material adjacent to a slit, or between parallel slits about a plane of the fin to a prescribed angle. Such processes may be cumbersome to manufacture and confer relatedly adverse manufacturing economics. This arises because, under traditional approaches, many punching stations are needed to sheer the fin strip in order to define the louvers. This step may produce waste material in the form of scrap fragments that can diminish the life of a forming dye.
Also, there is a need to make such exchangers competitively, while reducing waste material, improving heat energy dissipation characteristics and prolonging the life of the manufacturing equipment necessary to make the heat exchanger apparatus.
Among the relevant prior art are these references: EP0430852; EP0384316; U.S. Pat. Nos. 4,984,626; 4,561,494 and 5,036,911, the disclosures of which are incorporated by reference.